Mobile Wizard Assault Platform:
1) Twin internally locking access hatchs, allows quick entry and egress regardless of orientation of platform, while preventing assailant from reaching wizard inside.
2) External walls have lip which slightly overlaps underlying forceplate. Lips have L-shaped bars on pivots such that they may be locked around underside of plate, and fastened from inside. This means that fortress cannot be tipped without tipping plate as well. It also means that the interior space can be greater in diameter than the diameter of the forceplate.
3) Fortress is ringed with blades to allow fortress to be pressed againt attackers in close quarters.
4) Internal screw jacks allow fortress to be elevated to a height of 1', so as to renew ritual in field.
5) A tower shield is suffient to provide nearly total protection for a person of average height. It weighs 45lbs. It stands to reason that six barriers of similar size could provide an enclosure, with a weight of around 300 lbs. Counting equipment, a dome, and other features here described, the total weight of the structure wouldn't exceed 500lbs. One of great enough diameter to give an archer room to fire probably wouldn't exceed 800lbs.
So using the floating disk to provide a platform for a floating mantlet seems doable.
The way to get this past a DM is to use the platform in the totally expected 'reasonable' way on a test run. Use it to ferry treasure, or a wounded friend - something that the DM 'wants' you to be able to do. If the disk can follow you up staircases and across normal woodland terrain, you are in good position to commission your tensor's floating fortress from local craftsman.